OHSKKVATKiNS OX VISION. 258 



(Tlic wi'ilcr (.'oniKH'ts wilh lliis phciioiiicnon, which was hrsl, oh- 

 M'r\('<l hy \'()n Bozohl and later iiioi'c exactly im^st i<i,at(Hl hv IIcrin|Li' 

 and i lillchrand. the oriii-iii of the tlu'orics of Iviiiiio- and Yow Kric^s.) 



\l. TIIK riKKlXJK I'lIKNOMENOX WITH I,AK(;F, AXI) S.MAI, L FIKLDS. 



{(() L(n'(/e felds. — A projection ai)i)aratns with two lai'<2;e Nicol 

 j)risnis is suitahle for the denionsti'ation of tlu> Pnrkinje plienomenon. 

 For this purpose the slit is rephiced hy a round opening, and an 

 enlarged image is thrown upon the Aarious coh)red fields to be coin- 

 ]»are(h These are preferably of colored paper, from which foui' 

 arcs are combined to form a hirge field. Red and blue-green are 

 suitabli' colors and are to Ix' so chosen that with the strongest avail- 

 able lighting the red is perceived to be more bi-ight than the blue- 

 green. In case suitable papers are not available, the same effect can 

 be reached by interposing in the beam suitably colored glasses or 

 stained gelatin films. If the brightness of the light is gradually 

 diminished, the blue-green becomes at length l)righter than the red, 

 and finally the red disappears, while the blue-green changes to the 

 colorless •' rod-white." 



(I>) Ohsei-ration of sukiU p'cJds. — The large field just described 

 is covered with black i)a])er or velvet, except in two small spots, where 

 the colored paper may be seen. Now, however nuich the intensity 

 of the light is diminished, the red always appears brighter than the 

 l>lue-green, provided both spots are observed directly so that their 

 images fall on the fovea centralis or macula lutea, and not toward the 

 ])eriphery of the retina where both i-ods and cones are present. 



(f) AlfertKife ohserratioii of flic large <ind small fields. — In order 

 to uuike the competition of the rods and cones still more striking, the 

 two fields may be observed alternately. Having reduced the inten- 

 sity of the light on the small field, all the while recognizing the red 

 to b(> ])riglitei- than the blue-green, the screen is remo\-ed and the eye 

 sees, again, suddenly, the large field. .The blue-green [)arts now 

 iippear colorless. l)ut of a magical brightness, while the red j)arts 

 Ix'come almost black. 



The Purkinje i)henomenon may be ()bsei-\-ed in ordinary conditions, 

 as when a |)icture gallei'v is visited on a \i'1t cloiuly day or in the 

 twilight, for then all the red tints ap])ear dai'k and obscui'e, and all 

 the blue ones coloi'U'ss and whitish." 



Helmholtz also mentions that of all objects the blue sky appeared to 

 retain color longest at twilight, and I believe I am right in assuming 

 that the moonlit landsca[)e is '' rod-white," for at least the silver bis- 

 ter of nu)<)idight is vei-y similar to the ghostly gray-white of the rods. 



•"IT. V. Ilelniholt/-: llandbucb der Physiol. Optik. 2 ed. p. 429. (Leopold 

 Voss, Loiiizig. 1896.) 



